How long do u cook 12 pound turkey
Epicurious has a great chart for turkey doneness, taking into account turkey size, oven temp & time: http://bit.ly/dMQo7M But it's best to take the internal temperature too -- you'll want to hit at least 150 in the fleshy part of the thigh (the temp will continue to climb as it rests).
If you are in the UK, use this guide
15 to 20 minutes per pound at 325 F. You really need an instant meat thermometer. 185 F thigh, 170 F breast.